Conventionally, an indexable rotary cutting tool is used as a tool for forming tooth spaces of a gear. This tool includes a tool body having insert mounting seats disposed on both side end surfaces of the tool body, and a cutting insert is attached with a tightening screw from a side end surface side to each of the insert mounting seats. A cutting tooth form of the tool is the same as at least a part of a tooth space of the gear, and by machining the tooth space with the tool, the cutting tooth form is transferred directly to a work material as the shape of the tooth.
Patent Literature 1 discloses an example of the cutting insert used in a rotary cutting tool for a gear cutting as described above. As illustrated in FIG. 1A to FIG. 1C, in this cutting insert, an intersecting ridgeline portion between an upper surface and a side surface is formed as a cutting edge; the side surface serves as a rake face; the upper surface serves as a flank; and a lower surface serves as an insert seating surface. The cutting edge is convex toward the upper surface side in a side view (see FIG. 1C) of this cutting insert viewed from the side surface on which the cutting edge is formed at the edge portion. Thus, this upper surface has a curved shape that corresponds to the convex shape of the cutting edge. According to a description of Patent Literature 1, the cutting edge is formed into the shape of an involute tooth profile that is machined with a rotational path of the tool body around the axis.